Extending SystemVerilog Data Types to Nets
SystemVerilog extended Verilog by adding powerful new data types and operators that
can be used to declare and manipulate parameters and variables. Extensions like packed
structs provide a very convenient abstraction for manipulating an object that is really just
a bit vector.
SystemVerilog did not extend these new data types to nets. However, with the addition of
continuous assignments to variables, hardware designers can use the extended data types
with variables to model many common network behaviors. Users would like to have
these convenient abstractions for nets too, because other common network behaviors —
bidirectionality, multiple driver resolution, and delays — cannot be modeled with
variables.
We propose to extend SystemVerilog by making a subset of the new data types available
for nets too. In this first IEEE revision of SystemVerilog, we would like to allow a net or
port to have any fixed-size data type that is based on four-state logic. You can use new
SystemVerilog data types to declare parameters and variables, and by extension you can
use new data types to declare nets too. For example:
typedef struct packed { logic ecc; logic [7:0] data; } MemLoc;
wire MemLoc memsig;
This extension for nets is limited to four-state data types because of schedule constraints
in proposing LRM changes. We would have liked to propose two-state extensions as
well. It is not our intent to preclude the extension of other data types to nets in the future.
Data Objects and Data Types
You can look at Verilog data objects as having two primary characteristics.
One is the "kind" of the object (i.e., variable vs. parameter vs. net) and the other is the
"data type" of the object (integer vs. real vs. scalar bit, etc).
Roughly, the object kind indicates what you can do with the object. Only parameters can
be modified with defparam statements, only variables can be assigned by procedural
assignments,only nets have values that are resolved from their drivers, etc.
Roughly, the data type of an object indicates the values the object can take on. A data
object of type 'real' can take on the value 3.14, an object of a bit vector type can take on
the value 4'b0xz1, etc.
These two characteristics of a data object are largely orthogonal. As examples, a variable
can be of any data type, and a bit vector can be the data type of any kind of data object.
Certain kinds of data objects have additional characteristics. For example, a net has a "net
type", such as wire or trireg, that determines how its value is computed.
The diagram below illustrates these concepts. This diagram shows the relationships
between a data object and its significant properties. A data object is a construct that has a
name and a value associated with it. Thus, an important characteristic of a data object is
the set of values that it can have — that is, its "data type". Certain kinds of data objects
have additional properties of interest. For example, a variable is also characterized by its
lifetime, and a net is also characterized by its net type.
Diagram 1: Characteristics of a Verilog Data Object
static
automatic
bit
01
Variable
logic, reg
01XZ
Parameter
integer
… -1 0 1 …
Localparam
array
…
Specparam
real
…
Attribute
enum
…
Genvar
class
…
struct
…
Net
…
wire
tri
wand
triand
wor
trior
supply0
supply1
tri0
tri1
trireg
Proposed SystemVerilog LRM Changes
This section proposes a set of specific LRM changes to extend SystemVerilog to allow
all fixed-size four-state data types on nets. The basis for this extension is covered in
sections 3 (Data types), 5 (Data declarations), and 18 (Hierarchy). These core changes are
proposed first, followed by a set of changes throughout the LRM to provide a consistent
view of data types and data objects.
Annex A.2.1.3
CHANGE:
net_declaration ::=
net_type_or_trireg [drive_strength|charge_strength] [vectored|scalared]
[signing] {packed_dimensions} [delay3] list_of_net_decl_assignments;
TO:
net_declaration ::=
net_type_or_trireg [drive_strength|charge_strength] [vectored|scalared]
data_type_or_implicit [delay3] list_of_net_decl_assignments;
In Syntax 18-4 and
Annex A.2.2.1
CHANGE:
port_type ::= [net_type_or_trireg] [signing] {packed_dimension}
TO:
port_type* ::= [net_type_or_trireg] [signing] {packed_dimension} |
[net_type_or_trireg] data_type
*When a port_type contains a data_type, it shall only be legal to omit
the explicit net_type_or_trireg when declaring an inout port.
3.1 Introduction
CHANGE:
Verilog-2001 has net data types, which can have 0, 1, X, or Z, plus 7 strengths, giving
120 values. It also has variable data types such as reg, which have 4 values 0, 1, X, Z.
These are not just different data types, they are used differently. SystemVerilog adds
another 4-value data type, called logic (see Sections 3.3.2 and 5.5).
TO:
Verilog-2001 has data objects that can take on values from a small number of predefined
value systems: the set of four-state logic values, vectors and arrays of logic values, and
the set of floating point values. SystemVerilog extends Verilog by introducing some of
the data types that conventional programming languages provide, such as enumerations
and structures.
In extending the type system, SystemVerilog makes a distinction between an object and
its data type. A data type is a set of values and a set of operations that can be performed
on those values. Data types can be used to declare data objects, or to define user-defined
data types that are constructed from other data types.
The Verilog-2001 logic system is based on a set of four state values: 0, 1, X, and Z.
Although this four-state logic is fundamental to the language, it does not have a name.
SystemVerilog has given this primitive data type a name, logic. This new name can be
used to declare objects and to construct other data types from the four-state data type.
CHANGE:
Verilog-2001 provides arbitrary fixed length arithmetic using reg data types. The reg type
can have bits at X or Z, however, and so are less efficient than an array of bits, because
the operator evaluation must check for X and Z, and twice as much data must be stored.
SystemVerilog adds a bit type which can only have bits with 0 or 1 values. See Section
3.3.2 on 2-state data types.
TO:
Verilog-2001 provides arbitrary fixed length arithmetic using 4-state logic. The 4-state
type can have bits at X or Z, however, and so may be less efficient than an array of bits,
because the operator evaluation must check for X and Z, and twice as much data must be
stored. SystemVerilog adds a bit data type that can only have bits with 0 or 1 values. See
Section 3.3.2 on 2-state data types.
5.1 Introduction
CHANGE:
There are several forms of data in SystemVerilog: literals (see Section 2), parameters (see
Section 21), constants, variables, nets, and attributes (see Section 6)
TO:
There are several forms of data in SystemVerilog: literals (see Section 2), parameters (see
Section 21), constants, variables, nets, and attributes (see Section 6). A data object is a
named construct that has a data value associated with it, such as a parameter, a variable,
or a net.
ADD TO END OF SECTION:
SystemVerilog extends the set of data types that are available for modeling Verilog
storage and transmission elements. In addition to the Verilog-2001 data types, new
predefined data types and user-defined data types can be used to declare constants,
variables, and nets.
5.2 Data declaration syntax
ADD TO SYNTAX BOX:
net_declaration ::=
net_type_or_trireg [drive_strength|charge_strength] [vectored|scalared]
data_type_or_implicit [delay3] list_of_net_decl_assignments;
New section "5.5 Nets", right after section 5.4 Variables
A net declaration begins with a net type that determines how the values of the nets in the
declaration are resolved. The declaration can include optional information such as delay
values and drive or charge strength.
Verilog-2001 restricts the data type of a net to a scalar, a bit vector, or an array of scalars
or bit vectors. In SystemVerilog, any four-state data type can be used to declare a net. For
example:
trireg (large) logic #(0,0,0) cap1;
typedef logic [31:0] addressT;
wire addressT w1;
wire struct packed { logic ecc; logic [7:0] data; } memsig;
If a data type is not specified in the net declaration then the data type of the net is logic.
Certain restrictions apply to the data type of a net. A valid data type for a net shall be one
of the following:
1. A four-state integral type
2. An unpacked array or unpacked struct, where each element has a valid data type
for a net
The effect of this recursive definition is that a net is comprised entirely of four-state bits,
and is treated accordingly. There is no change to the Verilog-2001 semantics related to
net resolution at the bit level, the role of strength, or the treatment of the signed property
across hierarchical boundaries.
A lexical restriction applies to the use of the reg keyword in a net or port declaration. A
Verilog net type keyword shall not be followed directly by the reg keyword. Thus, the
following declarations are in error:
tri reg r;
inout wire reg p;
The reg keyword can be used in a net or port declaration if there are lexical elements
between the net type keyword and the reg keyword.
18.1 Introduction
CHANGE:
An important enhancement in SystemVerilog is the ability to pass any data type through
module ports, including nets, and all variable types including reals, arrays and structures.
TO:
An important enhancement in SystemVerilog is the ability to pass a value of any data
type through module ports, using nets or variables. This includes reals, arrays and
structures.
18.8 Port Declarations
CHANGE:
With SystemVerilog, a port can be a declaration of a net, an interface, an event, or a
variable of any type, including an array, a structure or a union.
TO:
With SystemVerilog, a port can be a declaration of an interface, an event, or a variable or
net of any allowed data type, including an array, a structure or a union.
CHANGE:
If the first port direction but no type is specified, then the port type shall default to wire.
This default type can be changed using the `default_nettype compiler directive, as in
Verilog."
TO:
If the first port direction but no net type or data type is specified, then the port shall
default to a net of net type wire. This default net type can be changed using the
`default_nettype compiler directive, as in Verilog.
CHANGE:
For subsequent ports in the port list, if the type and direction are omitted, then both are
inherited from the previous port. If only the direction is omitted, then it is inherited from
the previous port. If only the type is omitted, it shall default to wire. This default type
can be changed using the `default_nettype compiler directive, as in Verilog.
// second port inherits its direction and type from previous port
module mh3 (input byte a, b);
...
endmodule"
TO:
For subsequent ports in the port list, if the direction and the net type and data type are
omitted, then the direction and any net type and data type are inherited from the previous
port. If the direction is omitted, but a net type or data type is present, then the direction is
inherited from the previous port. If the direction is present, but the net type and data
types are omitted, then the port shall default to a net of net type wire. This default net
type can be changed using the `default_nettype compiler directive, as in Verilog.
// second port inherits its direction and data type
// from previous port
module mh3 (input byte a, b);
...
endmodule
For an inout port, if the net type is omitted, then the port shall default to a net of net type
wire. This default net type can be changed using the `default_nettype compiler directive,
as in Verilog.
// the inout port defaults to a net of net type wire
module mh2 (inout integer a);
...
endmodule
18.11.3 Instantiation using implicit .name port connections
DELETE:
- A port connection between a net type and a variable type of the same
bit length is a legitimate cast.
18.12 Port connection rules
CHANGE:
SystemVerilog extends Verilog port connections by making all variable data types
available to pass through ports.
TO:
SystemVerilog extends Verilog port connections by making values of all data types on
variables and nets available to pass through ports.
18.12.2 Port connection rules for nets
CHANGE:
If a port declaration has a wire type (which is the default), or any other net type,
TO:
If a port declaration has a net type, such as wire,
CHANGE:
- An output can be connected to a net type (or a concatenation of net types) or a
compatible variable type (or a concatenation of variable types).
- An inout can be connected to a net type (or a concatenation of net types) or left
unconnected, but not to a variable type.
TO:
- An output can be connected to a net or variable (or a concatenation of nets or variables)
of a compatible data type.
- An inout can be connected to a net (or a concatenation of nets) of a compatible data
type, or left unconnected, but cannot be connected to a variable.
18.12.4 Compatible port types
CHANGE:
The same rules for assignment compatibility are used for compatible port types for ports
declared as an input or an output variable, or for output ports connected to variables.
TO:
The same rules are used for compatible port types as for assignment compatibility.
3.6 chandle data type
CHANGE:
The chandle data type represents storage for pointers passed using the DPI Direct
Programming Interface (see Section 27). The size of this type is platform dependent, but
shall be at least large enough to hold a pointer on the machine in which the tool is
running.
TO:
The chandle data type represents storage for pointers passed using the DPI Direct
Programming Interface (see Section 27). The size of a value of this data type is platform
dependent, but shall be at least large enough to hold a pointer on the machine in which
the tool is running.
3.7 String data type
CHANGE:
SystemVerilog includes a string data type, which is a variable size, dynamically allocated
array of bytes. SystemVerilog also includes a number of special methods to work with
strings.
TO:
SystemVerilog includes a string data type. The values of the string data type are
dynamically allocated arrays of bytes of arbitrary size. SystemVerilog also includes a
number of special methods to work with strings.
4.2 Packed and unpacked arrays
CHANGE:
Packed arrays can only be made of the single bit types (bit, logic, reg, wire, and the other
net types) and recursively other packed arrays and packed structures.
TO:
Packed arrays can be made of only the single bit data types (bit, logic, reg) and
recursively other packed arrays and packed structures.
New section 5.8.1, Matching types, approved for erratum 254
CHANGE:
3) An anonymous enum, struct, or union type matches itself among variables
declared within the same declaration statement and no other types.
TO:
3) An anonymous enum, struct, or union type matches itself among data objects
declared within the same declaration statement and no other data types.
CHANGE:
4) A typedef for an enum, struct, union, or class matches itself and
the type of variables declared using that type within the scope of the type identifier.
TO:
4) A typedef for an enum, struct, union, or class matches itself and the type of data
objects declared using that data type within the scope of the data type identifier.
5.8.1 Equivalent types
CHANGE:
3) An anonymous enum, struct, or union type is equivalent to itself among
variables declared within the same declaration statement and no other
types.
TO:
3) An anonymous enum, struct, or union type is equivalent to itself among
data objects declared within the same declaration statement and no other
data types.
CHANGE:
4) A typedef for an enum, unpacked struct, or unpacked union, or a class
is equivalent to itself and variables declared using that type within
the scope of the type identifier.
TO:
4) A typedef for an enum, unpacked struct, or unpacked union, or a class
is equivalent to itself and to data objects that are declared using
that data type within the scope of the data type identifier.
7.3 Assignment operators
CHANGE:
The semantics of such an assignment expression are those of a function which evaluates
the right hand side, casts the right hand side to the left hand data type, stacks it, updates
the left hand side and returns the stacked value. The type returned is the type of the
left hand side data type. If the left hand side is a concatenation, the type returned shall be
an unsigned integral value whose bit length is the sum of the length of its operands.
TO:
The semantics of such an assignment expression are those of a function that evaluates the
right hand side, casts the right hand side to the left hand side data type, stacks it, updates
the left hand side and returns the stacked value. The data type of the value that is returned
is the data type of the left hand side. If the left hand side is a concatenation, then the data
type of the value that is returned shall be an unsigned integral data type whose bit length
is the sum of the length of its operands.
7.12 Concatenation
CHANGE:
SystemVerilog enhances the concatenation operation to allow concatenation of variables
of type string. In general, if any of the operands is of type string, the concatenation is
treated as a string, and all other arguments are implicitly converted to the string type (as
described in Section 3.7). String concatenation is not allowed on the left hand side of an
assignment, only as an expression.
TO:
SystemVerilog enhances the concatenation operation to allow concatenation of data
objects of type string. In general, if any of the operands is of the data type string, the
concatenation is treated as a string, and all other arguments are implicitly converted to the
string data type (as described in Section 3.7). String concatenation is not allowed on the
left hand side of an assignment, only as an expression.
CHANGE:
The replication operator (also called a multiple concatenation) form of braces can also be
used with variables of type string. In the case of string replication, a non-constant
multiplier is allowed.
TO:
The replication operator (also called a multiple concatenation) form of braces can also be
used with data objects of type string. In the case of string replication, a non-constant
multiplier is allowed.
7.16 Aggregate expressions
CHANGE:
Unpacked structure and array variables, literals, and expressions can all be used as
aggregate expressions.
TO:
Unpacked structure and array data objects, as well as unpacked structure and array
constructors, can all be used as aggregate expressions.
7.17 Operator overloading
CHANGE:
The overload declaration allows the arithmetic operators to be applied to data types that
are normally illegal for them, such as unpacked structures. It does not change the
meaning of the operators for those types where it is legal to apply them. This means that
such code does not change behavior when operator overloading is used.
TO:
The overload declaration allows the arithmetic operators to be applied to data types that
are normally illegal for them, such as unpacked structures. It does not change the
meaning of the operators for those data types where it is legal to apply them. This means
that such code does not change behavior when operator overloading is used.
CHANGE:
The overload declaration links an operator to a function prototype. The arguments are
matched, and the type of the result is then checked. Multiple functions can have the same
arguments and different return types. If no expected type exists because the operator is in
a self-determined context, then a cast must be used to select the correct function.
Similarly if more than one expected type is possible, due to nested operators, and could
match more than one function, a cast must be used to select the correct function.
TO:
The overload declaration links an operator to a function prototype. The arguments are
matched, and the data type of the result is then checked. Multiple functions can have the
same arguments and different return data types. If no expected data type exists because
the operator is in a self-determined context, then a cast must be used to select the correct
function. Similarly if more than one expected data type is possible, due to nested
operators, and could match more than one function, a cast must be used to select the
correct function.
CHANGE:
An expected result type exists in any of the following contexts:
TO:
An expected result data type exists in any of the following contexts:
CHANGE:
The overloading declaration links the + operator to each function prototype according to
the equivalent argument types in the overloaded expression, which normally must match
exactly. The exception is if the actual argument is an integral type and there is only one
prototype with a corresponding integral argument, the actual is implicitly cast to the type
in the prototype.
TO:
The overloading declaration links the + operator to each function prototype according to
the equivalent argument data types in the overloaded expression, which normally must
match exactly. The exception is if the actual argument is an integral type and there is only
one prototype with a corresponding integral argument, the actual is implicitly cast to the
data type in the prototype.
9.5 Continuous assignments
CHANGE:
SystemVerilog removes this restriction, and permits continuous assignments to drive nets
any type of variable.
TO:
SystemVerilog removes this restriction, and permits continuous assignments to drive nets
and variables of any data type.
11.5 Object properties
CHANGE:
Any data type can be declared as a class property, except for net types since they are
incompatible with dynamically allocated data.
TO:
There are no restrictions on the data type of a class property.
19.2: Interface syntax
CHANGE:
The aim of interfaces is to encapsulate communication. At the lower level, this means
bundling variables and wires in interfaces, and can impose access restrictions with port
directions in modports.
TO:
The aim of interfaces is to encapsulate communication. At the lower level, this means
bundling variables and nets in interfaces, and can impose access restrictions with port
directions in modports.
23.4 Expression size system function
CHANGE:
The $bits function can be used as an elaboration-time constant when used on fixed sized
types; hence, it can be used in the declaration of other types or variables.
TO:
The $bits function can be used as an elaboration-time constant when used on fixed sized
types; hence, it can be used in the declaration of other data types, variables or nets.
23.7: Array querying system functions
CHANGE:
SystemVerilog provides system functions to return information about a particular
dimension of an array variable or type.
TO:
SystemVerilog provides system functions to return information about a particular
dimension of an array data object or data type.
ANNEX J Glossary
ADD:
Data object - A data object is a named construct that has a data value associated with it,
such as a parameter, a variable, or a net. A data object has a data type that determines
which values the data object can have.
ADD:
Data type - A data type is a set of values and a set of operations that can be performed on
those values. Examples of data types are logic, real, and string. Data types can be used to
declare data objects, or to define user-defined data types that are constructed from other
data types.
CHANGE:
Aggregate - An aggregate expression, variable or type represents a set or collection of
singular values. An aggregate type is any unpacked structure, unpacked union, or
unpacked array data type.
TO:
Aggregate - An aggregate expression, data object or data type represents a set or
collection of singular values. An aggregate data type is any unpacked structure, unpacked
union, or unpacked array data type.
CHANGE:
Bit-stream - A bit-stream type or variables is any type that can be represented as a serial
stream of bits. To qualify as a bit-stream type, each and every bit of the type must be
individually addressable. This means that a bit-stream type can be any type that does not
include a handle, chandle, real, shortreal, or event.
TO:
Bit-stream - A bit-stream data type is any data type whose values can be represented as a
serial stream of bits. To qualify as a bit-stream data type, each and every bit of the values
must be individually addressable. This means that a bit-stream data type can be any data
type except for a handle, chandle, real, shortreal, or event.
CHANGE:
Dynamic - A dynamic type or variable is one that can be resized or re-allocated at
runtime. Dynamic types include those that contain dynamic arrays, associative arrays,
queues, or class handles.
TO:
Dynamic - A dynamic data type or variable has values that can be resized
or re-allocated at runtime. Dynamic arrays, associative arrays, queues, class handles and
data types that include such data types are dynamic data types.
CHANGE:
Enumerated type - Enumerated data types provide the capability to declare a variable
which can have one of a set of named values. The numerical equivalents of these values
can be specified. Enumerated types can be easily referenced or displayed using the
enumerated names, as opposed to the enumerated values. Section 3.10 discusses
enumerated types.
TO:
Enumerated type - Enumerated data types provide the capability to declare a data object
that can have one of a set of named values. The numerical equivalents of these values can
be specified. Values of an enumerated data type can be easily referenced or displayed
using the enumerated names, as opposed to the enumerated values. Section 3.10 discusses
enumerated types.
CHANGE:
Integral - An integral expression, variable or type is used to represent integral, or integer
value They may also be called vectored values. Integrals may be signed or unsigned,
sliced into smaller integral values, or concatenated into larger values.
TO:
Integral - An integral data type represents integral, or integer, values. Integral values
may also be called vectored values. Integral values may be signed or unsigned, sliced into
smaller integral values, or concatenated into larger values. An integral expression is an
expression of an integral data type. An integral data object is an object of an integral data
type.
CHANGE:
Singular - A singular expression, variable or type represents a single value, symbol, or
handle. A singular type is any type except an unpacked structure, unpacked union, or
unpacked array data type.
TO:
Singular - A singular expression, data object or data type represents a single value,
symbol, or handle. A singular data type is any data type except an unpacked structure,
unpacked union, or unpacked array data type.